Many gas turbine engine fuel supply systems include a fuel source, such as a fuel tank, and one or more pumps that draw fuel from the fuel source and deliver pressurized fuel to the fuel manifolds in the engine combustor via a main supply line. The main supply line may include one or more valves in flow-series between the pumps and the fuel manifolds. These valves generally include, for example, a main metering valve and a pressurizing-and-shutoff valve downstream of the main metering valve. In addition to the main supply line, many fuel supply systems also include a bypass flow line connected upstream of the metering valve that bypasses a portion of the fuel flowing in the main supply line back to the inlet of the one or more pumps, via a bypass valve. The position of the bypass valve is typically controlled by a head regulation scheme to maintain a substantially fixed differential pressure across the main metering valve.
The above-described fuel supply system is generally safe, reliable, and robust. Nonetheless, it can suffer certain drawbacks. For example, the metering valve and bypass valve can add to overall system weight and complexity. Moreover, control of the metering valve and bypass valve can result in increased system complexity and cost. Thus, in recent years there has been a desire to implement more direct metering fuel control systems. In such systems fuel flow rate is controlled by controlling, for example, the speed or the displacement of the fuel metering pump. Yet, efforts to implement direct fuel metering control systems have also been impeded by the certain drawbacks. For example, many gas turbine engines include one or more secondary fuel loads, such as one or more fluid-operated actuators. In many instances these actuators are driven by the fuel from the fuel supply system. Thus, actuator operation can cause a droop in the fuel supplied to the engine, and thus an undesirable engine speed droop. It is also postulated that such fuel supply variations to the engine could yield unpredictable, and potentially less controllable, engine transients.
The above-mentioned drawbacks may be addressed by including an independently driven servo pump to supply adequate flow and pressure to the secondary fuel loads. This solution, however, exhibits its own drawbacks. In particular, the independent servo pump and its associated drive mechanism (e.g., a motor) and control mechanism (e.g., controller) increase overall system weight and costs. Yet another solution, which is disclosed in U.S. patent application Ser. No. 11/706,910, and which is assigned to the assignee of the instant invention, is to implement a software compensation algorithm in the fuel metering pump controller to compensate fuel metering pump speed to supply adequate flow and pressure to the secondary fuel loads. This solution also exhibits drawbacks in that it may rely on the use of a pressurizing valve, and the number of secondary loads that may be compensated for may be limited.
Hence, there is a need for a system and method of supply adequate fuel flow to secondary fuel loads in a direct metering fuel control system that does not increase overall system weight and costs and/or that is not limited in the number of secondary loads for which compensation may be provided. The present invention addresses one or more of these needs.